Solid state motor speed control

ABSTRACT

A solid state motor speed control is described that is capable of controlling a motor so that it operates quietly, eliminating annoying buzzing noises that would otherwise be generated by the motor&#39;s windings. The invention provides for motor speed control with infinitely variable speed control settings. Rather than use triacs that generate RFI noise and cause motor windings to buzz due to their inherent avalanche switching, the invention utilizes power FETs or IGBTs to provide the switching function to control power delivery to the motor load. The power switches are controlled by an oscillator that generates a pulse train with a duty cycle controllable by the user. A filter shapes this signal into a trapezoidal control signal that drives the gate input of the power FET or IGBT switch. A kick start circuit turns the motor on initially for two to three seconds to overcome the inertia of the motor and its load. A zero crossing detector prevents the motor from being turned on except at a zero crossing of the AC source to further reduce noise.

This application is a continuation of U.S. application Ser. No.08/183,460, filed Jan. 18, 1994, now abandoned.

BACKGROUND OF THE INVENTION

In practice today there are basically two ways to control the speed ofan AC motor. The first way is to reduce the amplitude of the AC voltagereaching the motor. The lower the amplitude of the AC voltage, theslower the motor runs. The amplitude can be reduced by placing aresistor in series with the motor and the AC line. The problem with thismethod is that the series resistor dissipates large amounts of power asheat. Instead, tapped auto-transformers, tapped motor windings or seriescapacitors are used. However, it is impossible to create infinitelyvariable versions of this type of motor speed control. The difficultyand expense involved prevent the manufacture of auto-transformers ortapped motor windings with a large number of taps. Instead, most ofthese types of motor speed controls come with a small number of fixedfactory preset taps or steps. Thus, a user is limited to typically twoor three speeds only i.e. low-high or low-medium-high.

The second way to control the speed of an AC motor is to switch the ACto the motor on and off so that the voltage applied across it ischopped. The more voltage chopped out of the AC line, the slower themotor runs. To achieve infinitely variable motor speed control, themajority of today's controls employ a phase shifted triac type ofdesign. These controls work by varying the amount of time the AC isapplied to the motor during each half cycle. A solid state switch builtaround a triac device is typically connected in series with the motorbeing controlled. At the beginning of each half cycle the triac is offor open. If the triac is turned on at a high phase angle or late in theAC cycle then the motor will be powered for a short time. Conversely, ifthe triac is turned on early in the cycle or at a low phase angle thenthe motor will be powered for most of the time and turn very fast. Thevoltage applied to the motor is a phase shifted chopped sine wave.

The problem with using a triac device as a motor speed control is thatit only has two states, on and off. Triacs exhibit switching timesmeasured in nanoseconds causing huge inrush currents to flow through themotor. These inrush currents cause the motor windings and the metallaminations adjacent to them to contract and expand, producing arelatively loud audible noise. The noise is loudest at half speed whenthe triac turns on at a phase angle of 90°. At this point, the AC to themotor switches from zero to maximum voltage. An unpleasant buzzing noiseis emitted with a large harmonic content of 120 Hz since the triacswitches during every half cycle of the 60 Hz AC source. If theapplications utilizing a triac switch are noisy to begin with, i.e. handdrills, food processors, etc., the loud buzzing noise is tolerated ordrowned out by the motor itself. However, there are applications wherequiet operation is essential such as in the control of ceiling fans.Most users of ceiling fans would find the 120 Hz buzzing of the motorwindings unpleasant and annoying.

In addition to their noise problem, another drawback of using triacs forswitching control of fan motors is the large amount of Radio FrequencyInterference (RFI) they generate. A triac, with its very fast switchingtimes, generates RFI because its switching waveform resembles an idealstep function in time when it is turned on and off. The ideal frequencyresponse of this step function includes components from the entirefrequency spectrum. Thus, triacs emit RFI over a wide frequencyspectrum.

Other problems associated with conventional motor speed controls arethat they take up much space and dissipate large amounts of heat.Especially large mounts of heat are dissipated in designs utilizing aresistor placed in series with the motor.

Therefore there is a long felt need for a solid state motor speedcontrol that is capable of infinitely variable speed control thatproduces a minimum of audible noise and RFI. In addition, it should besmall enough to fit in a standard wall box receptacle and dissipate anegligible amount of heat. To achieve these goals a different type ofsolid state switch is utilized. Instead of a triac, with its drawback ofavalanche switching, power Field Effect Transistors (FET) or InsulatedGate Bipolar Transistors (IGBT) are used which exhibit linear ratherthan avalanche switching. Power FETs and IGBTs are voltage controlledimpedances whereas triacs are current controlled switches. By applying asuitable control signal to a power FET or IGBT, such as a trapezoidalshaped voltage signal, the motor can be made to turn on and off slowlythereby limiting the high inrush currents which causes the loud annoyingbuzzing associated with designs incorporating triacs.

A ceiling fan motor speed control circuit utilizing IGBTs has been shownto achieve its highest power efficiency and smoothest fan rotation whenthe motor is switched on and off with a period of 0.1 to 10 seconds.This relatively low switching frequency is not noticeable to the userbecause of the large amount of momentum inherent in ceiling fans whilethey are spinning. It is this inertia or resistance to acceleration thatmasks the on and off switching of the motor to the user.

SUMMARY OF THE INVENTION

A primary object of the present invention is to provide a motor speedcontrol such that the motor operates quietly and a user is not subjectedto an annoying buzzing noise.

Another object of the present invention is to provide for infinitevariability of motor speed control settings so that any desired settingfrom minimum to maximum speed is obtainable.

Yet another objective of the present invention is to minimize both theamount of RFI generated and the heat dissipated when compared withconventional motor controls.

Another objective of the present invention is that it be constructedwith solid state components and small enough to fit in a standard wallbox receptacle.

Yet another objective of the present invention is that it be a twoterminal device operating from one side of the AC source only andconnected serially between one side of the AC source and the motor load.

These objectives are achieved by the present invention which may bebroadly characterized as a motor speed control comprising switchingmeans for providing continuous linear variation of the flow ofelectrical current through a load and speed control means electricallyconnected to the switching means for generating a control signal to turnthe switching means on and off repeatedly, determining the amount ofelectrical current that flows through the switching means.

BRIEF DESCRIPTION OF THE DRAWINGS

Serving to illustrate exemplary embodiments of the invention are thedrawings of which:

FIG. 1 is a functional block diagram of the preferred embodiment of thepresent invention;

FIG. 2 is a detailed schematic diagram of the preferred embodiment ofthe present invention;

FIG. 3 is a wiring diagram showing the typical wiring configuration ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

In order to afford a complete understanding of the invention and anappreciation of its advantages, a description of a preferred embodimentof the present invention in a typical operating environment is presentedbelow.

Shown in FIG. 1 is a functional block diagram of the quiet solid statemotor speed controller 10. The motor speed controller 10 is a twoterminal device that is normally placed in series with a load, typicallybeing a ceiling fan incorporating a motor 124. The hot phase side of theAC source 22 is connected directly to the LINE terminal of the motorspeed controller 10. The MOTOR terminal is connected to one side of themotor 124 and the AC neutral is connected to the other side of themotor. A mechanical switch 120 turns power on and off to the motor speedcontroller 10. An oscillator 18 provides the timing and control signalsthat ultimately turn the voltage controlled switch 128 on and off. Thevoltage controlled switch 128 allows current to flow through the motor124 when it receives a control signal from the gate filter 20 telling itto turn on. A frequency feedback control circuit 126 adjusts the signalfrom the oscillator 18 so as to keep the fan motor 124 running smoothlyand quietly. A kick start circuit 12 provides the motor 124 withincreased power during the initial two to three seconds of operation toovercome the relatively large inertia of the motor and the mechanicalload attached to it. The gate filter 20 shapes the oscillator 18 signalin a such a way as to reduce motor noise. A DC power supply 16 providesDC voltage to the circuitry of the controller 10. A zero crossingdetector 14 prevents the oscillator 18 from turning on the voltagecontrolled switch 128 at a time other than at the point of a zerocrossing of the AC cycle.

In FIG. 2 is shown a detailed schematic diagram depicting the motorspeed controller 10 circuitry. The DC power supply 16 includes diodes37, 36 to provide half wave rectification of the AC power source 22.This rectified voltage charges capacitor 42 through resistor 38. Arelatively high DC voltage of 100 V develops across storage capacitor42. The high DC voltage charges capacitor 46, a large storage capacitor,through resistor 40 and clamped by zener diode 44 to a voltage V_(DD) ofapproximately 12 V. Voltage V_(DD) is the voltage provided to power thecontroller 10 low voltage circuitry. The power supply 16 consists of twostages rather than one to be able to provide power to the controller 10when the motor is running within a few percent of its maximum speed. Atany speed, when the motor is on, the voltage controlled switch 128 opensand closes every AC half cycle. When the motor is set to run at maximumspeed, the voltage controlled switch 128 is closed for most of thecycle, therefore there is a limited amount of time available forcharging capacitor 42. The power supply 16 has the burden of providingpower to the controller 10 circuitry during the time the voltagecontrolled switch 128 is closed. Therefore, the storage capacitors 42,46 must be sufficiently large to hold adequate charge when the switch128 is closed. During the time switch 128 is closed, capacitor 42discharges through resistor 40 and charges capacitor 46. This preventsthe output supply voltage V_(DD) from dropping when the duty cycle ofthe motor is high. In addition, the small RC time constant of resistor38 and capacitor 42 allows very quick charging of capacitor 42 duringthe short time the switch 128 is open.

The oscillator 18 is built around a Schmitt inverter 58 with a feedbacknetwork consisting of capacitor 48, variable resistors 64, 72, diodes70, 66 and resistor 68. When the output of the inverter 58 is high,capacitor 48 charges through variable resistor 72 (R₇₂), diode 70 andthe portion of variable resistor 64 between the common terminal anddiode 70 (R_(64A)). Charging continues until the voltage acrosscapacitor 48, which also is input to the inverter, reaches the thresholdvoltage V_(T+) causing the inverter 58 to switch to the low state. Thetime the inverter 58 output is high can be expressed as ##EQU1## Inputvoltage V_(T-) is the threshold voltage below which the inverterswitches to the high state. When the inverter 58 output switches low,capacitor 48 discharges through resistor 68 (R₆₈) and the portion ofvariable resistor 64 between the common terminal and diode 66 (R_(64B)).Discharging continues until the voltage across capacitor 48 falls belowV_(T-) which causes the inverter 58 output to switch to the high state.The time the inverter 58 output is low can be expressed as ##EQU2## Thetime of one complete cycle is given by ##EQU3## and

    R.sub.64A +R.sub.64B =R.sub.64 =total resistence of variable resistor 64

The time for one cycle can then be expressed as ##EQU4## The equationspresented above for t_(on), t_(off) and the inverse of frequency areonly approximate due to the voltage drops across diodes 70, 66 whilecapacitor 48 is charging and discharging.

Variable resistor 64 provides infinitely variable adjustment of theinverter 58 output duty cycle without needing to change the frequency ofthe oscillator. Low RPM variable resistor 72 provides adjustment for theminimum t_(on) which determines the minimum speed of the motor 124.Different motors might require different minimum speed settings. Usercontrol of the motor 124 speed is infinitely variable, from the lowestpower, slowest speed to the highest power, fastest speed.

The gate filter 20 filters the oscillator 18 output control voltagesignal and applies it to the voltage controlled switch 128. Componentsof the gate filter 20 include resistors 74, 78, 80 and capacitors 76,82, 84. The pulse output of the oscillator 18 is shaped by the low passRC network of resistor 74 and capacitor 76. A trapezoidal shaped voltagewaveform develops across capacitor 76 as it charges and dischargesthrough resistor 74. The output voltage of this RC network drives thecontrol inputs of the voltage controlled switch 128. Resistors 78, 80are necessary to prevent parasitic oscillations or chatter within thevoltage controlled switch 128. Capacitors 82, 84 are optional beingconnected to the control inputs of the switching devices in the voltagecontrolled switch 128. These capacitors can be used to further definethe shape of the signal applied to the control inputs of the switch 128.

The voltage controlled switch 128 consists of two solid state switches86, 88. Any type solid state switch with a linear switching region issuitable such as Junction Field Effect Transistors (JFET), InsulatedGate Bipolar Transistors (IGBT) or Darlington transistors. Triacs,however, are not suitable because they only have two states, on and off.With switching times measured in nanoseconds, triacs cause undesirablehigh inrush currents to the motor 124, causing excessive noise to begenerated in the motor's windings. Linear sold state switches do notemploy avalanche switching and therefore do not create motor noise. Theswitches may be used individually with a bridge providing DC voltageacross them or in pairs in a totem pole arrangement. In the preferredembodiment the switch 128 consists of two FETs 86, 88 connected sourceto source in series with the motor 124 being controlled. Two FETs arerequired because each FET can only switch power in one direction.Inherent in the physics of any power FET device is a reverse diode thatallows current to flow through the device when a FET is reverse biased.During the positive half cycle of the AC power source 22, switch 86 ison but switch 88 conducts through its reverse diode even though it isoff. Conversely, during the negative half cycle of the AC source 22,switch 86 conducts through its reverse diode and switch 88 is on.

The connection of the sources of the two switches 86, 88 creates afloating ground between them. This floating ground becomes a referencefor the DC power supply 16 and the gate voltages of the FET switches 86,88. The floating ground permits both FETs 86, 88 to be turned on and offsimultaneously by a single drive control signal. Thus, control of theFETs 86, 88 is independent of the momentary phase angle of the AC source22. This is not true, however, in the control of triacs.

The kick start circuit 12 provides a solution to the problem that existswhen the motor 124 is initially turned on and set at a very low speed.In this case, the motor 124 might not receive enough current to overcomethe inertia of the motor's 124 rotor and the mechanical load attached toit such as the blade assembly of a ceiling fan. By forcing the motor 124to run at full speed for 2 to 3 seconds this problem is overcome and apositive start is assured. When power is first applied to the controller10, voltage V_(DD) appears across capacitor 24. The output of inverter28 is low and the output of inverter 30 is high. Diode 32 is forwardbiased and provides a positive voltage across capacitor 76. This causesthe voltage controlled switch 128 to turn on at full power. Capacitor 24slowly charges up, reducing the voltage across resistor 26 until, afterabout 2 to 3 seconds, it falls to a point where it causes the output ofinverter 28 to be switched high. This causes inverter 30 to switch lowremoving any positive voltage across capacitor 76 thereby turning offthe motor 124. Diode 32 decouples the kick start circuit 12 from theoscillator circuit 18 so it no longer has any effect on the solid stateswitch after the inverter 30 switches low.

As previously described, motor 124 speed is controlled by changing theduty cycle of the oscillator 18 signal. At high motor 124 speeds, therelatively low frequency oscillator 18 signal is adequate to control fanspeed with imperceptible motor jerkiness. However, at low speeds the lowfrequency oscillator 18 signal causes the motion of the fan to becomenoticeably jerky. To solve this problem, the frequency feedback controlcircuit 126 increases the oscillator 18 frequency at low fan speeds.This removes any jerkiness in the fan rotation. Lowering the oscillator18 frequency only during high fan speeds also serves to achieve greaterpower efficiency since the heat dissipation of the controller 10 isdirectly proportional to frequency.

The frequency feedback control circuit 126 consists of resisters 60, 52,capacitors 62, 56, diode 50 and Darlington transistor 54. Inverter 58charges capacitor 62 through resistor 60 when its output is high. Whenthe duty cycle is low, the charge developed across capacitor 62 throughresistor 60 is insufficient to turn on transistor 54. At this point,capacitor 56 is not a part of the oscillator 18 circuit and theoscillator 18 is forced to oscillate at a higher frequency, eliminatingperceptible jerky fan motion. However, at high duty cycles, the outputof inverter 58 is able to charge capacitor 62 through resistor 60sufficiently high enough to turn on and saturate transistor 54. Currentflows through capacitor 56 which is switched into the oscillator 18circuit. The addition of this parallel capacitor 56 to the oscillator 18circuit causes the oscillator 18 to oscillate at a lower frequency whenthe duty cycle is high (i.e. fast motor 124 speeds) thereby decreasingcontroller 10 power dissipation. Resistor 52 slows down the charging anddischarging of capacitor 56 providing a gradual change in frequency fromslow to fast motor 124 speeds. Diode 50 prevents capacitor 56 fromdriving the collector of the transistor 54 negative with respect toground, limiting the lowering of the oscillator 18 frequency at high fanspeeds.

To assure quiet motor 124 operation and to eliminate the generation ofRFI, a zero crossing detector 14 prevents the motor 124 from beingswitched on at any time in the AC cycle other than at the zero crossing.Diodes 90, 92, 94, 96 form a full wave bridge rectifier between theMOTOR and LINE terminals. Resistors 116, 118 limit the current throughthe bridge and in the event diodes 90, 92, 94, 96 fall, limit thecurrent that can flow between the MOTOR and LINE terminals. The voltageacross load resistor 98 is clamped to approximately voltage V_(DD) bydiode 122. The waveform across load resistor 98 is the unfiltered ACsource 22 with a peak of approximately V_(DD) referenced to the floatingground. This waveform is input to the Schmitt inverter 100 which outputsa steady low with the exception of short V_(DD) pulses at each zerocrossing of the AC source 22. The inverter 100 output is applied to thegate filter 20 and voltage controlled switch 128 through resistor 110and diode 114. Diode 114 decouples the zero crossing detector 14 fromthe gate filter 20 when the voltage across capacitor 76 is low.

When the output of the oscillator switches high and the output of theinverter 100 is low, the voltage divider formed by resistors 74, 110prevent capacitor 76 from charging to a high enough voltage to turn onthe voltage controlled switch 128. When the output of the inverter 100switches high, capacitor 76 can charge through resistor 74 as normal.The switch 128 closes and the motor 124 is turned on. Thus, turn on ofthe fan is delayed until the zero crossing of the AC voltage. This makesthe linear switching portion of FETs 86, 88 less critical, provides forquiet motor control and lowers controller 10 power dissipation.

During the time the switch 128 is closed, the voltage between the MOTORand LINE terminals drops to zero eliminating the generation of zerocrossing pulses. However the control circuitry is still powered fromstorage capacitor 46. Eventually the oscillator 18 switches low andcapacitor 76 starts discharging and the voltage controlled switch 128starts to open. The voltage between the MOTOR and LINE terminals risesand the output of inverter 100 switches low. To prevent noisy fan motoroperation caused by capacitor 76 discharging too quickly, the zerocrossing detector 14 operation is modified during the time the motor 124is being switched off. While the motor 124 is on, the output of inverter100 is high. Inverters 102, 104 charge capacitor 108 through resistor106 and diode 112 is reverse biased. When the output of inverter 100switches low again, the charge on capacitor 108 is added to the chargeflowing from capacitor 76 through resistor 106. The discharge ofcapacitor 76 is thereby slowed, preventing noisy fan motor operation.When the voltage across capacitor 108 falls to a low enough level it isdecoupled from the circuit by diode 112.

Shown in FIG. 3 is the wiring scheme for a typical application of themotor speed controller 10 connected to the AC source 22 and a ceilingfan incorporating a motor 124. The controller 10 is a two terminaldevice and connects to only one side of the AC source 22 in series withthe ceiling fan load. A user control knob 130 is connected internally tothe mechanical switch 120 and variable resister 64. Thus, the user cancontrol both on/off operation and motor speed adjustment using controlknob 130.

It is clear that the above description of the preferred embodiment in noway limits the scope of the present invention which is defined by thefollowing claims.

What is claimed is:
 1. A motor speed control system comprising an ACsource, a ceiling fan motor electrically connected to a phase terminalof said AC source and a motor speed controller, said motor speedcontroller comprising:solid state switching means including at least oneFET electrically connected between said ceiling fan motor and a neutralterminal of said AC source for providing substantially continuous,substantially linear variation of a flow of electrical current from saidAC source through said ceiling fan motor in proportion to a controlsignal applied to said switching means; low pass filter meanselectrically connected to said switching means for transferring saidcontrol signal thereinto, said low pass filter means removing highfrequency signal content from said control signal; speed control meansincluding oscillator means which is electrically connected to said lowpass filter means for generating and supplying said control signal tosaid switching means via said low pass filter means to turn saidswitching means on and off repeatedly thereby determining said flow ofelectrical current and wherein said speed control means turns saidswitching means on and off such that said current is delivered to saidmotor at a frequency independent of a frequency of said electricalcurrent; frequency feed back control means electrically connected tosaid oscillator means, said feed back control means functioning toincrease a frequency generated by said oscillator at low fan motorrotation speeds in order to remove jerkiness in the rotation of said fanmotor; zero crossing detector means electrically connected between saidAC source and said ceiling fan motor, said zero crossing detector meansalso electrically connected to said filter means for preventing saidactuation of said switching means during times other than a timesubstantially concurrent with a zero crossing of an AC signal generatedwithin said AC source; and kick start means electrically connected tosaid filter means for briefly allowing maximum electrical current toflow through said ceiling fan motor upon initial application of said ACsource to said motor speed control system.
 2. A motor speed control asrecited in claim 1 wherein said speed control means further comprisesfrequency feedback means for changing the frequency of said oscillatormeans according to the speed of said ceiling fan.
 3. A motor speedcontrol system comprising:an AC source; a ceiling fan motor electricallyconnected to a phase terminal of said AC source; and a motor speedcontroller, said motor speed controller comprising:solid state switchingmeans including at least one IGBT electrically connected between saidceiling fan motor and a neutral terminal of said AC source for providingsubstantially continuous substantially linear variation of a flow ofelectrical current from said AC source through said ceiling fan motor inproportion to a control signal applied to said switching device; lowpass filter means electrically connected to said switching means forremoving high frequency signal content from said control signal; speedcontrol means including oscillator means which is electrically connectedto said filter means for generating and supplying said control signal tosaid switching means via said low pass filter means to turn saidswitching means on and off repeatedly thereby determining said flow ofelectrical current, and wherein; said speed control means turns saidswitching means on and off such that said current is delivered to saidmotor at a frequency independent of a frequency of said electricalcurrent; frequency feed back control means electrically connected tosaid oscillator means, said feed back control means functioning toincrease a frequency generated by said oscillator at low fan motorrotation speeds in order to remove jerkiness in the rotation of said fanmotor; zero crossing detector means electrically connected between saidAC source and said ceiling fan motor, said zero crossing detector meansalso connected to said filter means for preventing said activation ofsaid switching means during times other than a time which issubstantially coincident with a zero crossing of an AC signal of said ACsource; and kick star means electrically connected to said filter meansfor briefly allowing maximum electrical current to flow through saidceiling fan motor upon initial application of said AC source to saidmotor speed control system.
 4. A motor speed control as recited in claim3 wherein said speed control means further comprises frequency feedbackmeans for changing the frequency of said oscillator means according tothe speed of said ceiling fan.